Gas assisted liquid atomizers, often termed "airblast nozzles", are well known means for dispersing a stream of liquid, such as a liquid fuel, in a stream of gas, such as air. Airblast fuel nozzles are common in gas turbine engine applications wherein it is desired to achieve rapid dispersion and combustion of the liquid fuel in a confined combustor volume.
U.S. Pat. No. 4,139,157 issued on Feb. 13, 1979 to Simmons shows an airblast fuel nozzle wherein successive coaxial flows of air, fuel, and additional air are combined and comingled so as to achieve the desired rapid dispersion and atomization of the liquid fuel. The Simmons disclosure recites very specific diametral relationships between the coaxially central airstream, the annular fuel stream, and the annular additional airflow in order to achieve maximum dispersion of the liquid fuel.
More specifically, Simmons states that the discharge opening for the axially central airstream must be less than the ultimate diameter of the annular fuel stream by an amount at least equivalent to the radial thickness of the annular fuel stream at maximum fuel flow. The disclosure of Simmons is typical of prior art airblast fuel nozzles wherein the initial discharge of the central pressurized gas stream is smaller in diameter than the outside diameter of the discharge opening of the concentric annular fuel stream.
Such prior art airblast nozzles, while effective in dispersing the liquid fuel, experience a significant drawback if the central gas discharge opening and the annular fuel discharge opening should for any reason be subject to a non-concentricity or other misalignment. Test results have established that even a slight variation in the concentricity of the central air and annular fuel discharge openings can result in a major non-uniformity in the mass flow of the dispersed fuel downstream of the nozzle. Such non-uniformities are especially undesirable in high performance gas turbine engines, frequently resulting in a localized overheating of the combustor liner, turbine vanes, or other structure, reducing service life and possibly degrading overall combustor and turbine durability and performance.
For example, in a typical airblast fuel atomizer having a radial thickness of the annular fuel stream on the order of 0.040 inches (0.100 cm), a non-concentricity between the central air discharge opening and the outer diameter of the annular fuel discharge opening of 0.004 inches (0.010 cm) result in an unacceptably high imbalance of the dispersed fuel. For such nozzles a typical acceptance tolerance of no more than 0.001 to 0.002 inches (0.003-0.005 cm) is necessary to insure uniform fuel dispersion. As will be familiar to those skilled in the art, such tolerances are difficult and expensive to maintain, especially in cast or welded nozzle structures.
What is needed is a fuel nozzle which is able to achieve satisfactory dispersion of a liquid and which is relatively insensitive to non-concentricity between the central gas discharge opening and the downstream annular fuel discharge opening.